The rapid development of electronic devices has increased market demand for electrochemical devices such as fuel cells, capacitors, electrochromic windows, and battery systems. In response to the demand for battery systems in particular, practical rechargeable lithium batteries have been actively researched. These systems are typically based on the use of lithium metal, lithiated carbon, or a lithium alloy as the negative electrode (anode).
Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells have consisted of a non-aqueous lithium ion-conducting electrolyte composition interposed between electrically-separated, spaced-apart positive and negative electrodes. The electrolyte composition is typically a liquid solution of lithium electrolyte salt in nonaqueous aprotic organic electrolyte solvent (often a solvent mixture).
The selection of electrolyte solvents for rechargeable lithium batteries is crucial for optimal battery performance and involves a variety of different factors. However, long-term stability, ionic conductivity, safety, and wetting capability tend to be the most important selection factors in high volume commercial applications.
Long-term stability requires that an electrolyte solvent be intrinsically stable over the battery's range of operating temperatures and voltages and also that it be either unreactive with electrode materials or that it effectively form a passivating film with good ionic conductivity. Ionic conductivity requires an electrolyte solvent that effectively dissolves lithium electrolyte salts and facilitates lithium ion mobility. From the viewpoint of safety, the characteristics of low volatility, low flammability, low combustibility, and low toxicity are all highly desirable. It is also desirable that the battery's electrodes and separator be quickly and thoroughly wetted by the electrolyte solvent, so as to facilitate rapid battery manufacturing and optimize battery performance.
Aprotic liquid organic compounds have been the most commonly used electrolyte solvents for lithium batteries. Often, compounds such as ethers or carbonic acid esters (carbonates) have been utilized, as these compounds typically share the desirable properties of oxidative stability at positive electrodes operating at less than about 4.4V vs. Li+/Li, low reactivity with lithium-containing negative electrodes, and a thermodynamically favorable interaction with lithium ions (which is manifested in the electrolyte composition as a high degree of dissociation of the anion and the lithium cation of the electrolyte salt).
The most commonly used aprotic organic electrolyte solvents for use in lithium batteries include cyclic esters (for example, ethylene carbonate, propylene carbonate, γ-butyrolactone), linear esters, cyclic ethers (for example, 2-methyltetrahydrofuran, 1,3-dioxolane), linear ethers (for example, 1,2-dimethoxyethane), amides, and sulfoxides. A mixed solvent is sometimes preferred, since the properties of the electrolyte composition (conductance, viscosity, etc.) and its reactivity towards lithium can often be ‘tailored’ to give optimum performance.
Less traditional solvents such as carboxylic acid esters, sulfoxides, sulfones, and sulfonamides have been used as electrolyte solvents with varying success. Sulfones are typically solids at room temperature. Sulfones such as tetramethylene sulfone (sulfolane) and ethyl methyl sulfone, however, have been used as electrolyte solvents. Dimethylsulfone has also been utilized, but, with a melting point of 107° C., its utility has been limited to batteries that operate at elevated temperatures (that is, at temperatures above which the electrolyte composition can be maintained in the liquid state).
Drawbacks to the use of conventional lithium battery electrolyte solvents are generally related to their low boiling points and high flammabilities or combustibilities. Some solvents, such as the cyclic carbonates ethylene carbonate and propylene carbonate, have boiling points above 200° C. However, many electrolyte solvents have boiling points that are substantially lower and have flash points less than 100° F. Such volatile solvents can ignite during catastrophic failure of a fully or partially charged battery that has undergone, for example, a rapid discharge due to a short circuit. Additionally, volatile solvents present difficulties in the preparation and storage of electrolyte compositions as well as in addition of the composition to the battery during the manufacturing process. Another common problem of some conventional electrolyte solvents is that they often have a surface energy that is too high to spontaneously wet the battery components.